The present invention relates to a water turbine provided with guide vanes and, more particularly, to a water turbine with three-dimensional vane type guide vanes. A vane construction, in which an outer profile and/or position in section perpendicular to the rotational axis thereof, of each vane forming the guide vanes changes to a direction of a rotation axis thereof, is referred to as the three-dimensional vane type.
A conventional water turbine will be explained hereunder, referring to FIGS. 20(a) and 20(b). FIG. 20(a) is a horizontal sectional view of the conventional water turbine and FIG. 20(b) is a sectional view taken along a line Axe2x80x94A of FIG. 20(a). The water turbine is formed of a casing 1, stay vanes 2, guide vanes 3, a runner 4, etc. Water enters the casing 1 and then flows in the runner through the stay vanes 2 and the guide vanes 3. The water flowed in the runner 4 rotates the runner 4 in a rotation direction 11. Upper and lower sides of the runner 4 are fixed to a band 8 and a crown 10, respectively. The crown 10 is fixed to a rotating shaft 14 and the runner 4 rotates about the center of the rotating shaft 14. Each vane of the guide vanes 3 rotates about the center of a rotating shaft 7.
All vanes of the guide vanes 3 are opened and closed at the same phase. The opening and closing of the vanes are effected by rotation of a guide ring 24 connected to the rotating shaft 7 by arms 25a, 25b. For the guides vanes 3, it is necessary to fully shutdown water when the operation is stopped. Therefore, usually, each vane of the guide vanes 3 has a shape of two-dimensional vane type. A vane construction in which the outer profile and position in each section perpendicular to the rotating shaft 7 each are the same as those in the other sections is referred to as the two-dimensional vane type.
One of subjects which are important to the performance of a water turbine is to prevent cavitation. Occurrence of cavitation inside the runner 4 causes problems such as reduction in efficiency, occurrence of vibration or noises, etc. In the case where cavitation is remarkable, the runner 4 is damaged and its exchange may be required in some cases. In the case of the water turbine in FIGS. 20(a), 20(b), since the curvature of the runner 4 on the side of the band 8 is large, the flow in this region is accelerated, whereby the pressure is lowered. Therefore, it is important to prevent cavitation from occurring at the inlet of the runner 4 on the side of the band 8.
As a first prior art concerning prevention of cavitation, there is JP A 59-82580 which discloses a construction in which a leading edge portion 9 of the runner 4 is inclined against a rotation direction 11 from the crown 10 side to the band 8 side.
As a second prior art concerning prevention of cavitation, there are JP A 51-72846, JP A 52-98841 and JP A 61-43977, each of which discloses three-dimensional vane type guide vanes each having a side shape of a parallelogram or a trapezoid. A construction in which a trailing edge of a sectional shape of an upper side of the guide vane 4 extends toward the runner 4 also is disclosed.
As a third prior art concerning prevention of cavitation, there is JP A 60-182361 which discloses a guide vane constructed so that an angle between a peripheral direction of the runner and a straight line connecting a rotation axis of the guide vane and a trailing edge becomes smaller on a lower side.
In the case of the first prior art, cavitation on the suction surface side of blade of the runner 4 can be prevented when a water level of a dam is high, that is, in a high head. However, cavitation on a pressure surface side of blade of the runner 4 when the water level is low, that is, in a low head can not be prevented. Therefore, it is difficult to take countermeasures in the case where operation conditions change due to lowering in water level of the dam. Further, in order to prevent cavitation on an existing water turbine, it is necessary to exchange a runner 4 of a high manufacturing cost. Further, since the runner 4 is a large-sized structural component, much time is needed for conveying and installing the runner, which is a cause of rising in cost.
In the case of the second prior art, the guide vanes 3 can not shutdown a water flow path.
In the case of the third prior art, the design becomes difficult because it is necessary to change a vane shape of the guide vane from an upper side to a lower side.
An object of the present invention is to provide a water turbine which is provided with guide vanes each having a shape of three-dimensional vane type easy to design and is able to shutdown and which is able to prevent cavitation occurrence at a low cost.
First of all, a cause that cavitation occurs at the inlet of a runner will be explained, referring to FIG. 21. Cu, Cv and Cw in FIG. 21 denote a rotating speed of the runner, an absolute speed of water flow and a relative water flow speed to the runner, respectively. A triangle formed by those three speed vectors is called a speed triangle at the inlet of the runner. FIGS. 21(a), (b) and (c) show speed triangles at time of low head, at time of rated operation head and at time of high head, respectively.
As shown in FIG. 21, at time of any heads other than (b) of rated speed operation head, an angle of water flowing in the runner does not meet an angle of a leading edge (tip portion) of blade of the runner 4. Therefore, there is the possibility that cavitation occurs. At time of low head in FIG. 21(a), cavitation is likely to occur on the pressure surface side 13 of blade of the runner 4. At time of high head in FIG. 21(c), cavitation is likely to occur on suction surface side 12 of blade of the runner. Such a change in angle of water flowing in the runner is known to occur generally in water turbines.
Usually, the possibility of cavitation occurrence is larger at time of high head in which a load to the runner becomes larger. However, there is also some possibility of cavitation occurrence at a time of a low head, according to design. In general, it is required for cavitation not to occur within a range of operation head of the water turbine. Therefore, for design of runner, it is required to prevent cavitation from occurring at a head at which the cavitation is most likely to occur.
By the way, an inflow angle of water to the runner is an average inflow angle in a peripheral direction of the runner. Precisely, in the vicinity of blade of the runner and at the other place, the inflow angle changes in the peripheral direction. The reason will be explained, referring to FIG. 22. Generally, as blades generating lift, wings of an air plane, runner blades of a water turbine, etc. are known. At tips of those blades or wings, as shown in FIG. 22, a direction of fluid flow changes so that the fluid flows in from the pressure surface side. This is because a kind of eddy layer called circulation is formed around the blade, and the direction of flow is bent, receiving induction speed from the eddy layer.
In this manner, in the case where a blade generating lift is inside the flow, the inflow angle of fluid changes, with an upstream side being influenced by the blade. In the case of runner of a water turbine, also, as shown in FIG. 23, there is a tendency that water is flowed in from the pressure surface side 13 in the vicinity of the leading edge (tip end) 4a of blade of the runner 4. From the above-mentioned matters, also, it is found that cavitation is likely to occur on the suction surface side 12 of blade of the runner at time of high head. As shown in the same figure, a water flow at a place separate from the leading edge 4a of blade of the runner 4 is almost an average relative speed Cw.
An object of the present invention is to control a magnitude of the effect which blades of a water turbine runner generating lift impart to an upstream side thereof, based on the above-mentioned analysis result. This control changes a local inflow angle around the leading edge of blade of the runner and prevents occurrence of cavitation.
Concretely, in the case where the possibility of occurrence of cavitation is large on the side of band 8 when the head is low, a guide vane has a profile such that a distance between the runner becomes far (large) on the side of lower cover when the guide vanes are opened. By forming the guide vane in such a profile, an influence of the above-mentioned runner blade (blade generating lift) reaches to a more upstream side. As a result, the direction of a flow directed so as to be flowed in from the suction surface side 12 is bent to the pressure surface side 13 as shown in FIG. 21(a), whereby the inflow angle is caused to meet an angle of the leading edge 4a. 
In the case where the possibility of occurrence of cavitation is large on the side of the band 8 at time of high head, the guide vanes each are formed in such a profile that a distance between the runner becomes closer at the lower cover side when the guide vanes are opened. By making the guide vanes in such a profile, the above-mentioned influence of the runner blade (blade generating lift) does not reach to the upstream side. As a result, the flow is bent as shown in FIG. 23 and inflow from the side of the pressure surface 13 is prevented, whereby the inflow angle can be met with an angle of the leading edge 4a of the runner blade.
From the above consideration, a first invention to achieve the above-mentioned object resides in a water turbine which comprises a runner, guide vanes formed of a plurality of vanes arranged outside the runner in a peripheral direction, each of the vanes being rotatable about a center axis thereof parallel with a rotation shaft of the runner, stay vanes formed of a plurality of vanes outside the guide vanes in a peripheral direction, and a casing covering the outside of the stay vanes, wherein sections of each vane of the guide vanes perpendicular to the center axis are displaced to at least one direction of the peripheral direction and radial direction of the runner from one side to the other side in the center axis, under the condition that the guide vanes are shutdown, and respective profiles of the sections displaced are similar to each other and the size of each the section is set according to distance from the rotating shaft of the runner.
A second invention resides in a water turbine which comprises a runner, guide vanes formed of a plurality of vanes arranged outside the runner in a peripheral direction, each of the vanes being rotatable about a center axis thereof parallel with a rotation shaft of the runner, stay vanes formed of a plurality of vanes outside the guide vanes in a peripheral direction, and a casing covering the outside of the stay vanes, wherein sections of each vane of the guide vanes perpendicular to the center axis are displaced to the peripheral direction of the runner from one side to the other side in the center axis, under the condition that the guide vanes are shutdown.
A third embodiment is a water turbine which comprises a runner, guide vanes formed of a plurality of vanes arranged outside the runner in a peripheral direction, each of the vanes being rotatable about a center axis thereof parallel with a rotation shaft of the runner, stay vanes formed of a plurality of vanes outside the guide vanes in a peripheral direction, and a casing covering the outside of the stay vanes, wherein sections of each vane of the guide vanes perpendicular to the center axis are displaced to a radial direction of the runner from one side to the other side in the center axis, under the condition that the guide vanes are shutdown, and respective profiles of the sections displaced are similar to each other and the sections are formed so as to be smaller in size as distance from the rotating shaft of the runner becomes smaller.
A fourth invention resides in a water turbine which comprises a runner, guide vanes formed of a plurality of vanes arranged outside the runner in a peripheral direction, each of the vanes being rotatable about a center axis thereof parallel with a rotation shaft of the runner, stay vanes formed of a plurality of vanes outside the guide vanes in a peripheral direction, and a casing covering the outside of the stay vanes, wherein sections of each vane of the guide vanes perpendicular to the center axis are displaced to both a peripheral direction and a radial direction of the runner from one side to the other side in the center axis, under the condition that the guide vanes are shutdown, and respective profiles of the sections displaced are similar to each other and the sections are formed so as to be smaller in size as distance from the rotating shaft of the runner becomes smaller.
A fifth embodiment resides in a water turbine which comprises a runner, three-dimensional vane type guide vanes arranged outside the runner, stay vanes arranged outside the runner, and a casing covering the outside of the stay vanes, wherein each vane forming the guide vanes is formed so that a profile of the each vane, viewed from a radial direction of the runner is generally a parallelogram under the condition that the guide vanes are closed, and when the each vane is viewed from a center axis of rotation thereof, sections thereof perpendicular to the center axis appear deviated to a peripheral direction of the runner.
A sixth invention resides in a water turbine which comprises a runner, three-dimensional vane type guide vanes arranged outside the runner, stay vanes arranged outside the runner, and a casing covering the outside of the stay vanes, wherein each vane forming the guide vanes is formed so that a profile of the each vane, viewed from a radial direction of the runner is generally trapezoidal under the condition that the guide vanes are shutdown and when the each vane is viewed from the center axis of rotation thereof, sections thereof perpendicular to the center axis appear deviated to a radial direction of the runner.
According to the above-mentioned invention, under the condition that the guide vanes are opened at time of operation of the water turbine (hereunder, simply referred to as an opening condition), since distance between the guide vanes and the runner can be set most suitable according to a head or position at which cavitation is most likely to occur, occurrence of cavitation can be effectively prevented.
For example, in the case where cavitation is most likely to occur on the band side at time of high head, each vane of the guide vanes is formed so that sections of each vane of the guide vanes, perpendicular to a center axis of the each vane (hereunder, simply, referred to as sections of vane) are displaced to a rotation direction (peripheral direction) of the runner from the upper cover side to the lower cover under the condition that the guide vanes are shutdown (hereunder, simply, referred to as shutdown condition). Alternatively, the guide vanes can be formed so that under the shutdown condition of the guide vanes, sections of each vane of the guide vanes are displaced to a radially inner side of the runner from the upper cover side to the lower cover side.
By being provided with this construction, when the guide vanes are opened, distance between each vane of the guide vanes and each blade of the runner becomes small. As a result, since an influence of the blades of the runner does not reach to an upstream side as mentioned above, occurrence of cavitation can be effectively prevented.
In the case where cavitation is most likely to occur on the band side at time of low head, each vane of the guide vanes is formed so that under the shutdown condition of the guide vanes, sections of the each vane are displaced to an opposite direction (peripheral direction) to the rotation direction of the runner from the upper cover side to the lower cover side. Alternatively, the each vane of the guide vanes can be formed so that under the shutdown condition of the guide vanes, sections of each vane are displaced to a radially outer side of the runner from the upper cover to the lower cover.
By being provided with the present construction, when the guide vanes are opened, distance between the each vane of the guide vanes and each blade of the runner becomes larger on the lower cover side. As a result, an influence of the blades of the runner reaches to an upstream side as mentioned above, occurrence of cavitation can be effectively prevented.
In any above cases, respective profiles of sections which are displaced are sufficient if they are formed to be similar to each other and become small in size as the distance from the rotating shaft of the runner becomes small. In the case where the sections of vane are displaced to the peripheral direction of the runner, since the distance between each of the vane sections and the rotating shaft of the runner is the same, a similarity ratio is 1.
By forming the sections of each vane of the guide vanes in such sections, when the guide vanes are closed, a leading edge and trailing edge of adjacent vanes become in contact with each other. Therefore, shutdown performance of the guide vanes can be sufficiently secured.
Further, in any above-mentioned cases, vane sections are only displaced to the peripheral direction or radial direction of the runner while the similarity of profiles of the vane sections are being maintained. Therefore, if a vane profile of one section is designed, the other section can be obtained only by shifting the vane profile to the peripheral direction or radial direction while maintaining the similarity. That is, since it is unnecessary to redesign vane type itself in each section, the design becomes easy.
Further, according to the above-mentioned each invention, prevention of cavitation of an existing water turbine can be achieved by exchanging guide vanes. That is, since it is unnecessary to exchange a runner which is high in cost, cavitation can be prevented at a low cost.